The formation of hydrophilic coatings on substrates has many applications but in particular is most desirable in many biomedical applications. For example, biomedical applications such as wound drains, catheters, surgical tools and arteriovenous shunts, an article having a hydrophilic surface coating is desirable to minimize thrombosis, crystal formation, tissue trauma, tissue adhesion to surgical instruments, and foreign body reactions. In prior art methods, surfaces have been rendered hydrophilic by such methods as high energy radiation in situ polymerization processes, by direct chemical bonding or by forming interpolymer networks. The radiation process can render a very stable hydrophilic surface, but suffers from unreliable results and can produce radiation damage to the substrate. Formation of interpolymer networks also produces hydrophilic surfaces but in turbulent flow or extended soaking, the interpolymer networks often break down and the hydrophilic portion can be washed away rendering the substrate surface defective.
Prior art methods described using a polyurethane coating agent to adhere poly-N-vinyl pyrollidone (PVP) to various substrates, thus producing an article having a hydrophilic coating of low coefficient friction. Extensive studies indicate, however, that in turbulent flow or upon extended soaking in aqueous media, the hydrophilic coating can be leeched off, thus rendering the article insufficiently hydrophilic. The prior art depended upon non-covalent bonds and interpolymer networks produced to achieve coating stability. These methods of adhering a hydrophilic polymer such as PVP to a substrate surface coated with a urethane have not achieved the degree of coating stability required for many applications, particularly biomedical drains, catheters, and blood invasive devices.